Particularly a machine which operates in accordance with the thermodynamic Rankine cyclic process is understood by such a thermodynamic machine. The Rankine cyclic process in this case is characterized by pumping the liquid operating medium, by evaporating the operating medium at high pressure, by expanding the gaseous working fluid—performing mechanical work—and by condensing the gaseous working fluid at low pressure. Modern conventional steam power plants, for example, operate in accordance with the Rankine cyclic process. In fossil-heated steam power plants steam is typically produced with temperatures of over 500° C. at a pressure of over 200 bar. Condensing of the expanded steam takes place at about 25° C. and a pressure of about 30 mbar.
A thermodynamic machine operating in accordance with the Rankine cyclic process and also a method for the operation thereof is known from WO 2005/021936 A2, for example. Water serves as working fluid in this case.
If heat sources, which for the heat sink have only a relatively small temperature difference, are to be used for evaporating the working fluid, then the efficiency which can be achieved with the working fluid in the form of water is no longer sufficient for an economical mode of operation. Such heat sources, however, can be exploited with the aid of so-called ORC machines, in which instead of the working fluid in the form of water a low-boiling, especially organic fluid is used. From the point of view that such a fluid boils at lower pressures compared with water or has a higher vapor pressure in comparison to water, is understood by the term “low-boiling”. An ORC machine operates in accordance with the so-called organic Rankine cyclic process (ORC), i.e. basically with an especially organic, low-boiling working fluid which differs from water. As working fluids for an ORC machine, for example hydrocarbons, aromatic hydrocarbons, fluorinated hydrocarbons, carbon compounds—especially alkanes, fluoro ethers, fluoroethane—or even synthesized silicone oils are known.
By means of ORC machines or ORC plants, the heat sources available in geothermal or solar power plants, for example, can be economically used for power generation. Also, with an ORC machine it has been possible up to now for non-utilized waste heat of an internal combustion engine from exhaust air, cooling circuit, exhaust gas, etc., to be used for performing work or for power generation.
If the vapor pressure of a liquid which is associated with a respective temperature is fallen short of, this liquid evaporates. The falling short of the vapor pressure can take place in static or in moving liquids.
For example, in the case of a flowing liquid the vapor pressure can be locally fallen short of on account of a sharp deflection or acceleration of the flow so that a local evaporation takes place. The locally resulting vapor bubbles condense again at points of higher pressure and break down. The overall process is referred to as cavitation.
In a thermodynamic machine of the type referred to in the introduction, a cavitation which occurs in the liquid phase of the working fluid constitutes a not insignificant problem. On account of the small size of the vapor bubbles, the condensing of these takes place very quickly in fact. As a result of a sudden implosion of the vapor bubbles, a microjet is possibly formed in the process. If this is directed onto a surrounding wall, then pressure peaks of up to 10 000 bar can be locally achieved. In addition, as a result of the high pressures local temperatures of way above 1000° C. can be achieved, which can lead to melting processes in the wall material. Damage effects as a result of cavitations can occur within hours.
In a pump, the occurrence of cavitation, moreover, undesirably reduces the throughput of fluid. Since the vapor bubbles in their density as a rule differ considerably from the liquid, the deliverable mass flow is reduced even in the case of a low mass proportion of the working fluid as vapor at a given volumetric flow. In the event of a heavy build-up of vapor, the mass flow possibly even breaks down. If the working machine is used as a pump in an ORC plant, for example, then the entire cyclic process may possibly come to a standstill. As a result of the deficient pump output, a backing-up of the liquid working fluid in the condenser occurs, as a result of which its action is significantly reduced. As a result of this, the dissipation of heat comes to a halt. The overall system cannot easily be left in this state. A waiting period must be observed until the working fluid cools down by cooling of its own accord. In addition, the throughflow in the evaporator breaks down so that no heat can be dissipated any longer either. The working fluid which is used can then possibly be damaged as a result of exceeding its stability limit.
For a machine operating in accordance with the Rankine cyclic process, the problem of cavitation occurring is described in EP 1 624 269 A2, for example. There, a cavitation in the working fluid in the form of water inside the condenser and also inside the subsequent pump is to be prevented by a specific pressure and temperature control being provided at the condenser.
Corresponding pressure and temperature sensors are included for this. In particular, the water level in the condenser is maintained at a predetermined level. This is assisted by means of a drain valve which discharges water or non-condensing gases to the outside.
Also, the significance of a constant water level in the condenser for a machine operating in accordance with the Rankine cyclic process is described in U.S. Pat. No. 7,131,290 B2. Disclosed in particular is the effect of a variable water level upon the cooling surfaces in the condenser which come into effect. If non-condensing gas, such as air, penetrates into the cyclic system of the working fluid on account of the negative pressure conditions which prevail in the condenser, then this collects especially in the condenser. In order to prevent a loss of cooling capacity resulting therefrom, U.S. Pat. No. 7,131,290 B2 proposes a corresponding separation and drain device.
A complex fluid machine, which operates in accordance with the Clausius-Rankine cyclic process, is known from DE 10 2006 013 190 A1. The fluid machine has a pump for applying a pressure and for pumping out a liquid-phase working fluid, and an expansion device, connected in series to the pump, for creating a driving force by means of expansion of the working fluid which is heated in order to become a gas-phase working fluid. It is provided in this case to transfer the heat of the working fluid on an outlet side of the expansion device to the working fluid on an outlet side of the fluid pump.
A transportable drive unit for the conversion of heat, which is designed as a thermodynamic machine of the type referred to in the introduction and operates in accordance with the Rankine cyclic process, is known from DE 36 41 122 A1.
A steam power plant is known from DE 7 225 314 U, wherein an organic working medium is used in the Rankine cyclic process.
Also, a thermodynamic machine of the type referred to in the introduction is known from U.S. Pat. No. 4,291,232. In this case, a gas/liquid solution, especially an ammonia/water solution, circulates as working fluid.
By dissolution of the gas in the liquid, the pressure of the gas and liquid is lowered. By separating the gas under a temperature increase, the pressure is increased.